Wolf, Adam (Plant Sciences, Univ. of California, Davis, Dept of Plant Sciences, 1 Shields Ave, Davis, CA, 95616; Phone: 530-752-3450; Email: awolf@ucdavis.edu)

Spanning the Experimental Divide: Using Data at Multiple Spatial Scales to Better Constrain Gas Exchange Models

A. Wolf *, J. Six, C. Van Kessel, R. Howitt, D. Rolston, J. Hopmans, J. Mitchell

Understanding of surface greenhouse gas exchange at scales relevant to atmospheric science and climate policy remains a difficult topic: mass budgets are available only at very large or very small scales, but our typical scale of interest is somewhere intermediate. This paper describes the attempt to improve our understanding of greenhouse gas emissions at the field scale using both modeling and experimentation at multiple spatial and temporal scales. From 2003-2004, CO₂ and N₂O fluxes have been measured in a conventionally managed cornfield in Yolo County, CA. An eddy covariance system measured CO₂ and H₂O fluxes continuously over long periods over a several hectare footprint. In addition, closed chambers measured soil CO₂ and N₂O exchange weekly at 20-30 locations over the field. The EC measurements are rich in time, but only at one place; the CC measurements have rich spatial detail but are sparse in time. To span this spatiotemporal expanse, the biome model DNDC (“DeNitrification DeComposition”; Li, 2000) was parameterized to simulate components of greenhouse gas exchange both at the field scale and in a spatially distributed manner at the plot scale. Initial results show that the model can fairly simulate yield, but does not appear to simulate ecosystem (non-plant) respiration well, either before or during the growing season. Throughout the winter, CO₂ efflux measured by eddy covariance and closed chambers show much greater respiration than was modeled. Although growing season respiration is not directly measured by eddy covariance, the chamber measurements indicate that true respiration is much greater than modeled respiration throughout the growing period. Model estimate of nitrous oxide (N₂O) emission is well corroborated by the chamber measurements. Although N₂O efflux is very dynamic, the measurements match up well with both the baseline flux magnitude, as well as the peaks. This is heartening, because it shows that the model can reproduce the N₂O fluxes well even if its respiration estimates are biased. On balance, the model indicates that the Greenhouse Warming Potential (GWP, in CO₂ units) of N₂O efflux is a bit higher than for CO₂: 1151 kg CO₂ equivalents for CO₂ versus 4886 kg CO₂ equivalents for N₂O. Li C.S. (2000) Modeling trace gas emissions from agricultural ecosystems. Nutrient Cycling in Agroecosystems, 58, 259-276.